1. Field of the Invention
The present invention relates to a modulation device used when recording data on an optical disc and a method of the same and to a demodulation device used when reproducing the data from the optical disc and a method of the same.
2. Description of the Related Art
When recording a signal on an optical disc, the original signal is modulated to a signal suitable for recording on the optical disc.
As such a modulation system, there are for example the eight-to-fourteen modulation (EMF) system used for compact discs (CD) and mini discs (MD) and the 8-16 modulation system used in a digital video disc (DVD). In these modulation systems, however, the efficiency of the conversion (ratio of data length after modulation with respect to the data length before the modulation) is not that good (high). Namely, there is a high redundancy of data after modulation.
From the viewpoint of the conversion efficiency, the run-length modulation system, specifically, the RLL (1,7) modulation system, is excellent. The RLL (1,7) system modulates the original data to a minimum run of "1" and a maximum run of "7" bits of data.
In this RLL (1,7) modulation system, however, the low frequency component of the modulated signal is not controlled, so the low frequency component contained in the modulated signal is larger than that in the case of the EFM system, 8-16 modulation system, and other modulation systems which control the low frequency component.
Note that the low frequency component of the modulated signal is generally controlled by controlling the digital sum value (DSV). Here, the "DSV" is the sum of the symbols successively found from the point of time of the start of the waveform train when defining the symbols "1" and "0" in the recorded waveform train as "+1" and "-1", respectively.
When a low frequency component is contained in the modulated signal in this way, in a read only memory (ROM) disc on which data is recorded at pits on the disc, there is a problem in that this low frequency component will leak into the servo signal of an optical pick-up at the time of reproduction--which will have an adverse influence upon the servo control.
Further, if there is such a low frequency component, the demodulation circuit must be provided with a function for removing the influence of such a low frequency component, so there is a problem that the demodulation circuit becomes complex and expensive.
Note that, in a magneto-optical (MO) disc or other random access memory (RAM) disc, the servo control of the head is carried out by servo control marks formed on the disc, therefore the servo control is not influenced by the low frequency component contained in the reproduced signal.
In another matter, there are discs comprised of a ROM region for recording data by a pit train on the outside of which is provided a RAM region constituted by an MO disc etc. In such a disc, since the servo signal is not generated from a magnetic signal, it is not necessary to perform DSV control in the MO or other RAM region, but it is necessary to perform a certain degree of DSV control in the ROM region for the above reason. Here, a system may be considered where the DSV control is not performed when accessing the RAM region and is performed only when accessing the ROM region. In this system, however, It Is necessary to Individually provide a modulation circuit and demodulation circuit for accessing the RAM region and the ROM region, so the device becomes larger in size and higher in cost.
On the other hand, when DSV control is carried out for both of the RAM region and the ROM region, the redundancy of the data recorded in the RAM region becomes high, so the effective amount of data which can be recorded in the RAM region is reduced.